The present invention relates to a method and apparatus for checking and monitoring the accuracy of computerized or numerically controlled machine tools and in particular to a method and apparatus for continuously checking and monitoring the accuracy of the path of movement of the tool holding carriage of such machines during its normal operation in response to the preprogrammed computerized control.
Due to the complicated construction of the electronic circuits of computer or numerical control systems, the accuracy of the continued operation of the machine tool can be interfered with by external influences such as excessive vibration of the computer or controls or by the imposition of a stray voltage pulse from outside sources. In the event of such interference, incorrect data is processed and passed on to the machine carriage as a specific program order. Since the machine cannot distinguish between a correct or an incorrect order the machine is caused as a consequence to undertake a faulty movement. Such faulty movements frequently result in the destruction of costly tools or other parts of the machine itself as well as in damaging or destroying the work piece on which it is operating. Thus, not only do expensive parts of the machine have to be replaced, but the downtime of the machine during such replacements results in still further costs in production. This is a serious problem in the operation of machine tools since, particularly with the use of numerical control systems, the processing of incorrect data occurs irregularly and it is therefore difficult to predict and recognize in advance.
The most frequent cause of machinery damage occurs from an undesirable and wrong movement performed by the slideable carriages or movable tool holders. As a rule, the undesirable movement not only occurs in a fraction of a millimeter, but frequently, according to the decade of the control system being employed, up to a length of several millimeters. In fact, the undesirable movement frequently corresponds to the entire path of travel of the carriage. As a result, in addition to the disadvantages noted above, severe damage to the machine through collision of the moving parts frequently occurs.
For the above reasons, several methods have already been resorted to for increasing the functional safety of numerically controlled machine tools. One well-known measure consists of constructing the numerical control system in such a way that the punched tape coding containing the program is itself checked. To establish a standard the symbols which are stamped onto the punch strip have either an even or an odd number of perforations. In this manner the numerical control system is always capable of detecting an error when the even or odd number of the combination of punches does not correspond to the fixed standard. According to this system a reading error is indicated and a signal provided by which the control system is switched off. This system is based upon what is commonly referred to as "the parity check." The parity check insures that faulty movements of the machine do not occur when a particular order on the punch strip has a hole too many or too few. This system operates particularly advantageously when the punch strip is damaged or fouled so as to record an absence of a punch hole in a particular order. As a result of such damage the machine is brought to a standstill. The method of parity checking operates however only when one too many or one too few holes is detected. The system, however, does not distinguish between two holes and while the odd and even number rule is always adhered to, other defects and improper codings are not sensed and the machine is thus not switched off even when a faulty operation is taking place.
A further well-known measure provides for the repeated processing of the punch data in the punch strip reader at a predetermined frequency until a new punch order is read in. This repetition of the program reduces the likelihood of external interfering influences causing the transmission of a wrong data order to the machine tool. This type of control is, however, very expensive. Furthermore, it does not prevent the performance by the machine of an incorrect movement produced by an incorrect reading of the punch strip or by an error in the electronic system of the numerical control. As for example the failure of one of the electronic components. This type of system is described in the Swiss patent No. 480,915.
Another device is known from a report in the Journal "Werkstatt und Betrieb" No. 10 1969 by which the working program is simulated exterior of the machine and recorded by a drawing machine. In this system a visual indication of the working program is obtained during its initial run. However, errors occurring during the repetition of the program are neither sensed nor detected nor are they employed to control the machine operation.
Still another system employs a copying machine in which the shape of the work piece is defined as a length of a line which is scanned optically. The movement of the machine is controlled initially by the optical scanner and the machine is thus driven only as accurately as the drawing of the line and as accurately as the scanning system operates. The continuous line, of this system does not serve in this case to control the machine tool which receives a program from another source but to the contrary represents the program itself. Thus, disturbances in either the line or scanning system will lead to deviations in the program itself and cannot provide a selfmonitoring or checking system.
It is the object of the present invention to provide method and apparatus for operating a numerical control machine tool which overcomes the disadvantages of the prior art devices.
It is another object of the present invention to provide apparatus and method for checking and monitoring the path of movement of the tool carriage of a numerically controlled machine tool independently of the numerical control system or the initial operating program.
It is a further object of the present invention to provide a method and apparatus for checking and monitoring the path of movement of the movable member of a numerically controlled machine tool as a result of the direct movement of this movable member.
It is a further object of the present invention to provide method and apparatus for monitoring the path of movement of a numerically controlled machine tool so as to avoid expensive damage to the machine during its operation.
The foregoing objects, other objects, as well as numerous advantages of the present invention will be apparent from the following disclosure of the present invention.